In the wake of the Apple iPad announcement, Amazon has seen fit to offer its closest ever insight to how many Kindles it has shifted, something it has always kept a closely guarded secret.

Good news for those that are taking Amazon up on its Kindle Development Kit offer to make apps for the device, there are "millions" of Kindle customers out there, the etailer revealed as part of its most recent financial results statement.

"Millions of people now own Kindles", said Jeff Bezos, founder and CEO of in a statement.

"And Kindle owners read, a lot. When we have both editions, we sell 6 Kindle books for every 10 physical books. This is year-to-date and includes only paid books - free Kindle books would make the number even higher. It's been an exciting 27 months".

The Kindle market is now under threat from not only increasing numbers of competitors in the ebook reader segment, but also Apple's iPad tablet that will launch alongside Apple's new iBooks store, offering paid-for as well as free ePub titles.

When Apple launched the iPad and iBooks option, its CEO Steve Jobs nodded to the Kindle's seminal impact in the ebook market: "Amazon has done a great job of pioneering this... we're going to stand on their shoulders for this".

However, whether the iBooks/iPad combo stays standing on Amazon's shoulders or tramples it to the ground is yet to be seen.